DO PHYSICS ONLINE 1

DO PHYSICS ONLINE

QUESTIONS & PROBLEMS IDEAS TO IMPLEMENTION

How to answer a question: problem solving (t0_372.pdf)

View periodic table (cited Aug 2012)

CONSTANTS

electron charge e = |qe| = 1.60210-19

C

1 eV = 1.60210-19

J

electron mass me = 9.10910-31

kg

proton mass mp = 1.67510-27

kg

neutron mass mn = 1.67510-27

kg

speed of light c = 3.00108 m.s

-1

Planck’s constant h = 6.62610-34

J.s

magnetic force constant 2

ok

2.010

-7 N.A

-2

p1.23 08/12

The debate as to whether cathode rays are charged particles or electromagnetic waves continued

for many years. Which observation of cathode rays resolved this debate? Clearly distinguish the

wave and particle properties.

p1.28 07/24 (a) A negatively charged cylinder is

fixed in position near a positively charged

plate. Sketch the electric field lines

between the cylinder and the plate.

(b) A tiny particle of mass 10-30 kg and

charge +6×10-12 C is released at point Y.

The particle initially accelerates at 7.0×1021 m.s-2. Calculate the electric field intensity at Y.

DO PHYSICS ONLINE 2

p1.30 06/12 A charged non-magnetic particle is moving in a magnetic field. What factors affect the magnetic

force on the particle?

p1.34 4 An electron is moving near a long straight wire. When a current is applied to the wire the electron

experiences a force in the same direction as the current flow in the wire. What was the electron’s

initial direction of motion? Explain.

p1.44 08/23

Two parallel metal plates were in a magnetic field B

= 1.2310-3

T. The plates were separated by a

distance d = 22.5 mm. An electron was accelerated

by a 10.5 kV before entering the region between the

plates.

(a) What was the speed of the electron upon

entering the region between the plates?

(b) What was the magnitude and direction of the

force due to the magnetic field?

(c) What was the magnitude and direction of the force due to the electric field if the electron

continued on a straight path parallel to the plates?

(d) What was the magnitude and direction of the electric field between the plates?

(e) What was the voltage between the plates? Which plate was positively charged?

(f) How was this set-up used by J. J. Thompson used to measure the q/m ratio for an

electron?

p1.65 04/12

A discharge is shown.

(a) What type of pattern is shown?

(b) What end is the cathode?

(c) Explain the pattern observed?

(d) If the gas was changed, does the pattern change significantly?

(e) If the pressure was changed, does the pattern change significantly?

p1.68 01/2

A positively charged particle is moving with velocity v in

a magnetic field. At this moment, what is the direction of

the force on the positively charged particle? Explain.

Describe the subsequent motion of the particle.

uniform magnetic field out of page

B

ed

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p1.71 The diagram shows four discharge

tubes and the patterns of striations

observed in them.

(a) Explain the patterns.

(b) Account for the difference in the

striation patterns.

(c) List the tubes in order of

decreasing pressure.

p1.75 7

A charged non-magnetic particle is moving in a magnetic field. What would NOT affect the

magnetic force on the particle?

(a) The strength of the magnetic field.

(b) The magnitude of the charge on the particle.

(c) The velocity component parallel to the magnetic field direction.

(d) The velocity component perpendicular to the magnetic field direction.

Explain each alternative.

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p1.76 02/13

(a) What is the name of the device shown below?

(b) What is the purpose of the device?

(c) What is the name and function of the parts labeled A to F?

In a special investigation, the voltage between the cathode and the anode is

increased so that an electron gains a velocity of 0.60 c, where c is the speed of

light. The electron starts from rest at the cathode.

(d) What was the accelerating voltage for the electron gun?

The deflection plates are separated by a distance of 18 mm and the voltage across them is 254

V.

(e) What is strength of the electric field between the plates?

(f) What is the electric force acting on an electron?

(g) What acceleration of the electron?

p1.79 10

An electron travels at 2.0107 m.s

-1 in a plane perpendicular to a 0.0100 T magnetic field.

(a) Describe the path of the electron.

(b) Calculate the radius of the circular orbit.

(c) Calculate the period of motion.

(d) Calculate the frequency of the electron.

Hint: Centripetal force FC = m v2 / R

A

B

C

D

E

E

F

DO PHYSICS ONLINE 5

p1.93 1

A potential difference of 50 V is applied between two identical, parallel aluminum plates

which are separated by a distance of 10 mm. In order to double this electric field strength,

which new arrangement should be used?

Explain.

p1.94 K

Crookes developed a gas discharge tube to investigate the electrical nature of matter. When a

high voltage is applied to the tube, the glass behind the metal cross glows (fluoresces) and a

shadow of the cross appears.

(a) Explain the appearance of the shadow. If the cross is allowed to drop to the horizontal

position, describe and explain the pattern observed.

(b) Explain the fluoresces of the glass.

(c) When a magnet is moved towards the gas discharge tube, the shadow moves and is

distorted. Explain.

(d) How did the use of discharge tubes by scientists contribute to the understanding of

atomic structure?

(e) You have performed investigations using discharge tubes. Explain how you were able

to get them to function.

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p2.05 07/27

Describe the experiment used by Hertz and his observation of the photoelectric effect. What

did he do about these observations.

p2.06 Describe how Einstein used the ideas of Planck to explain the photoelectric effect.

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p2.08 An experiment was performed using a photocell. The surface was illuminated by light of

different frequencies and the stopping voltage was measured.

Define each of the terms: stopping voltage, threshold frequency, threshold wavelength, work

function of the surface.

(a) What is Einstein’s equation of conservation of energy explaining the photoelectric

effect in terms of the stopping voltage and frequency?

wavelength

(nm)

360 400 440 490 550 580

stopping

voltage (V)

1.45 1.12 0.95 0.60 0.40 0.25

(b) Plot the stopping voltage versus the frequency of the light.

(c) Determine: the threshold frequency, threshold wavelength, the work function in eV

and J, and the value of Planck’s constant.

(d) Another surface was used in the experiment. Its work function was 0.5 eV. Draw a

line on the graph for this surface.

p2.13 05/12

The family of curves below shows the

relationship between the intensity of black

body radiation and its wavelength for

various Kelvin temperatures.

Who was the first to correctly explain this

relationship?

What information do the graphs give you?

p2.33 07/27 Scientists tried to explain observations of blackbody radiation using classical wave theory and

then quantum theory.

(a) What is meant by the term blackbody radiation?

(b) What was a limitation of classical wave theory that could not explain blackbody radiation?

(c) How does quantum theory satisfactorily explain blackbody radiation?

p2.50 08/24

How did Einstein’s theory of special relativity and his explanation of the photoelectric

effect lead to the re-conceptualisation of the model of light?

DO PHYSICS ONLINE 7

p2.61 08/13

What is the energy of a photon (joules and electron volts) of wavelength 550 nm?

What is the frequency and what part of the electromagnetic spectrum for this photon?

p2.62 02/15

A student carried out an experiment during which light of different frequencies was

shone onto a metal surface to produce electrons. The student measured the maximum kinetic

energy of the emitted photoelectrons as the frequency of light was altered.

(a) How did the student measure the maximum kinetic energy?

(b) What is the mathematical relationship between the maximum kinetic energy of the

photoelectrons and the frequency of the light incident on the metal surface? Define

each term in the equation and its SI unit.

(c) How could the student best analyse the data to determine a value for Planck’s constant?

p2.66 K

Photoelectrons are emitted by a surface of a certain metal when the surface is illuminated by

both violet light of wavelength 400 nm and green light of wavelength 550 nm but no

photoelectrons are released from the surface by red light of wavelength 715 nm.

(a) Calculate the frequencies and the photon energies of the three light beams.

(b) Explain the differences in energies of the electrons released by the violet and green

light.

(c) Explain why no electrons are released when illuminated by red light.

p2.70 03/14

Heinrich Hertz used a set-up similar to the one shown to

investigate the production and detection of

electromagnetic radiation.

(a) In the diagram identify the transmitter, receiver,

transmitter, spark gaps, high voltage source.

(b) List the components of the electromagnetic

spectrum

(c) Explain the production and detection of the radio

waves.

A glass sheet was placed between the transmitter and receiver.

(d) Did the radio waves pass through the glass? Explain.

(e) Were ultraviolet waves blocked when the glass sheet was in place?

(f) Describe and explain the change in the spark length when the glass sheet was in place.

(g) How does this relate to the Photoelectric Effect?

p2.72 K

Assess the impact of the discovery of the photoelectric effect of the development of the

quantum model of light.

DO PHYSICS ONLINE 8

p2.90 05/23

Explain how an understanding of blackbody radiation changed the direction of

scientific thinking in the early twentieth century.

p2.95 06/26

Beginning in the late 19th century, observations and experiments on blackbody radiation and

the Photoelectric Effect led physicists to revise their existing model of light.

(a) What was the existing model for light?

(b) What was the evidence for this model?

(c) What model was introduced to explain blackbody radiation and the Photoelectric

Effect?

(d) Use the above as an example to explain how scientists test, validate and revise models.

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p3.13 08/15

A block of silicon doped with boron is connected to a 10 V battery.

What is the main way in which conduction occurs in the doped

silicon block?

p3.18 K Compare the accepted models that are used to describe how an electric current flows in a

metallic conductor at room temperature and a doped semiconductor at room temperature.

p3.22 K In terms of the band theory explain the essential conditions for a substance to be a

semiconductor.

p3.23 K Describe how p-type semiconductors are produced.

p3.24 K Discuss how shortcomings in available communications technology led to an increased

knowledge of properties of materials with particular reference to the invention of the

transistor.

p3.27 A solar cell can consist of an undoped silicon layer (~

1mm thick) placed between very thin (~ 2 nm thick) of

doped silicon.

(a) Describe how one of the doped layers can be made.

(b) What is the name of the effect that causes the

production of electrons in the undoped layer?

(c) Draw a labelled diagram to show how a potential difference is created across the cell by

the movement of holes and electrons.

12 V

n-Si

p-Si

undoped

Si

sunlight

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p3.30 02/14 During the early 1950s most transistors were manufactured using germanium.

(a) Why was germanium used instead of silicon?

(b) Why is silicon used today instead of germanium?

p3.33 07/14 (a) Summarize the property of a silicon p-type semiconductor? List a number of possible

doping atoms. (b) Summarize the property of a silicon n-type semiconductor? List a number of possible

doping atoms.

p3.39 04/25

An example of a solar cell is

shown. The solar cell is able

to produce a current due to

the photoelectric effect and

the electrical properties of

the n-type and p-type layers.

Use this information to

outline the process by which light shining on the solar cell produces an electric current that

can light up a light globe.

p3.44 07/22 04/23 Explain why solid state devices have largely replaced thermionic devices.

In the past 50 years electrical technology has developed from the widespread use of

thermionic devices to the use of solid state devices and superconductors.

List THREE disadvantages of thermionic devices that led to their replacement.

Outline ONE advantage of using superconductors, with reference to TWO

applications.

p3.52 06/23

(a) Draw labeled diagrams of the band structures of an insulator, a semiconductor, and a

conductor. With reference to your diagrams, describe the differences in electrical

resistance between insulators, semiconductors and conductors.

(b) Explain how the addition of trace amounts of certain elements, such as phosphorus,

can change the electrical resistance of semiconductors at a given temperature.

p3.55 Define the terms valance band and conduction band. Draw an energy level diagram for the

semiconductor and diamond.

If a semiconductor has a forbidden band gap of 0.8 eV, what is the maximum wavelength of

electromagnetic radiation that can be used to excite an electron from the valance band to the

conduction band in this material? What part of the electromagnetic spectrum does this

correspond to? Repeat the calculation of diamond, energy gap of 5.0 eV.

DO PHYSICS ONLINE 10

p3.57 Explain why the resistance of a semiconductor decreases as its temperature is increased but

for a metal it increases. Discuss conduction in metals, insulators and semicounductors.

p3.77 Assess the impact of the invention of the transistor on society.

p3.90 05/13

A doped silicon semiconductor has the

following energy-level diagram.

What element was most likely used to dope the

silicon?

p3.92 05/15

A current is passed along a square

semiconductor rod as shown. Half of the

current is carried by electrons and half by

holes. A magnetic field is then applied to

the rod at right angles to its axis. Describes

the movement of the electrons and holes in

the rod when the magnetic field is applied?

p3.95 05/25

A student conducts an experiment using a photoelectric cell. Light is shone through a grid

onto a metal surface. The metal is at earth potential and the grid is at 100 V, so that any

electrons emitted from the surface produce a current in the external circuit. The student

shines light sources of different photon energies onto the metal surface and records the

current flowing for each. The light sources are adjusted so that their intensities are equal. The

results are recorded in the table.

Photon energy (eV) Photo-current (μA)

0.5 0

0.9 0

1.2 0.5

1.70 2.8

1.75 4.0

1.90 8.0

2.20 9.2

2.50 9.4

(a) Plot the graph (X-axis – photon energy).

(b) Draw the straight line of best fit in the region where the photo-current varies greatest

with photon energy.

(c) From the line drawn on your graph, estimate the minimum energy (work function) for

photoelectric emission. What wavelength for the incident em radiation does this

correspond to and what part of the em spectrum?

(d) The experiment is repeated, but the intensities of the light sources are doubled. Predict

the results of this new experiment by drawing a second line on the graph.

(e) Explain the shape of the curves.

DO PHYSICS ONLINE 11

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p4.04 07/12 06/11 The Bragg experiment used X-rays to investigate the structure of crystals. (a) What property of waves was the basis of their technique?

(b) What single statement best describes the results of this experiment?

p4.14 K

Magnetic levitation is being investigated and tested for use in a number of areas including

transportation systems.

(a) What is meant by the term magnetic levitation

(b) Explain how magnetic levitation occurs in terms of the properties of the substances

involved.

(c) Describe the benefits and limitations of magnetic levitations as used in maglev trains.

p4.20 07/23 The table shows the critical temperature Tc at which some materials become superconducting.

(a) What is meant by the terms critical temperature and superconducting?

(b) What are the critical temperatures in oC?

(c) What are scientists working in the area of superconductivity trying to achieve?

p4.44 05/24

Explain how superconductivity occurs according to the BCS theory.

p4.49 08/14

The Meissner Effect occurs when a magnet is released above a superconductor that has been

cooled below its critical temperature and the magnet hovers above the superconductor. What

is the best explanation for this?

p4.50 03/23

Compare the model for the conduction of electricity in metals at room

temperature with the model for conduction of electricity in superconductors

below the critical temperature.

p4.60 02/12

Sketch a labeled graph to show how the resistance of a superconducting material changes as

its temperature drops below its critical temperature.

Year

discovered

Material Tc (K)

1941 Niobium nitride_ 16

1987 YBa2Cu3O7(YBCO)_ 92

1993 HgBa2Ca2Cu3O7 133

DO PHYSICS ONLINE 12

p4.75 06/21 (a) Superconductors offer a way of eliminating heating effects in many applications.

Evaluate the usefulness of using superconductors to eliminate energy losses in two

different applications.

(b) Assess the impact on society and the environment of the potential applications of

superconductors.

p4.78 The diagram shows two pieces of the same

superconducting material above and below its

critical temperature. The pieces are placed in a

strong magnetic field. Complete the diagram to

show the magnetic field in the region of both

pieces. Explain your completed diagram.

p4.80 Discuss the possible applications of using superconducting technology in computers.

p4.88 03/23

A magnet can hover above a superconducting disk.

(a) What is meant by a superconductor?

(b) Explain why the magnet is able to hover above the

superconductor.

(c) Name this effect.

(d) List two practical applications of the effect.

superconductors in a magnetic field

above critical

temperaturebelow critical

temperature

DO PHYSICS ONLINE 13

MISCELLANEOUS PROBLEMS

m008 01/25

A student carried out an

experiment on the photoelectric

effect. The frequency of the

incident radiation and the energy

of the photoelectrons were both

determined from

measurements taken during the

experiment.

(a) Graph the results.

(b) How could the reliability of the experiment be improved?

(c) The intensity of the incident radiation was tripled. How does it change the graph?

(d) Define the term critical frequency and estimate its value.

(e) Define the term work function and estimate its value.

m013 06/13 The temperature of a metal is reduced. What is the change in its electrical resistance and the

reason for this change? m020 02/27 Discuss how energy savings can be achieved in each of the two applications of

superconductors.

m045 05/14 An FM radio station transmits at a frequency of 102.8 MHz.

What is the energy, in joules and in eV of each photon emitted by the transmitter?

What is the wavelength of the transmitted signal.

m047 06/27

J. Plücker was the first to observe cathode rays within gas discharge tubes. He inferred that

the rays were a form of electromagnetic radiation.

(a) Describe a subsequent observation that led other scientists to argue that cathode rays

were charged particles.

(b) Identify a potential hazard associated with performing experiments with discharge

tube.

(c) Outline a safe work practice which addresses this hazard.

DO PHYSICS ONLINE 14

m049 05/27

Bubble chambers are used in conjunction with particle accelerators to photographically

record the tracks of fast-moving charged particles. An intense magnetic field is applied at

right angles to the path of the particles to deflect them according to their charge and

momentum. The diagram shows a beam of protons travelling horizontally at 6.00107 m.s

-1

and entering a liquid hydrogen bubble chamber in a vertical magnetic field of 1.82 T.

Examination of the photograph taken by the camera, as sketched below, shows that the

protons were deflected along a circular path of radius 0.350 m.

(a) Derive an expression for the momentum of a proton from the forces it experiences

in this experiment.

(b) Calculate the mass of a proton in the bubble chamber.

(c) Calculate the rest mass of a proton found from this experiment.

m050 05/11

The discharge tube shown below

contains a rotating paddle wheel that is

free to move.

The tube’s electrodes are connected to a

high-voltage source.

What does it tell you about cathode rays?

m065 A cathode ray tube and transistor circuits in a conventional television rely on transformers.

What type of transformer are needed for the cathode ray tube and transistor circuits? Explain.

DO PHYSICS ONLINE 15

m072 05/26 The diagram shows two

parallel horizontal metal

plates connected to a DC

source of

electricity. Suspended

between the plates is a

charged particle of mass

9.610-6

kg.

(a) Using conventional

symbols, draw the electric

field between the metal

plates on

the diagram above.

(b) Determine the magnitude of the electric field between the plates.

(c) Determine the sign and magnitude of the charge on the particle if it is suspended

motionless between the plates.

m076 Describe how an investigation can be performed to demonstrate the production and reception

of radio waves.

m080 The electric deflecting plates in a cathode ray tube are 12 mm apart and a potential difference

of 1234 V across them. A beam of positively charged particles moving at 2.67104 m.s

-1

travels through the plates undeflected.

(a) Sketch the experimental setup.

(b) Calculate the electric field between the plates.

(c) Calculate the magnetic field strength.

m082 08/21

The work of scientists is influenced by external factors. Do you agree? Justify your answer

with reference to the work of Einstein and Planck.

m088 04/14 The minimum amount of energy needed to eject an electron from a clean aluminum

surface is 8.72 × 10–19

J. What is the work function for aluminum in eV? What is the

maximum wavelength of incident light that can be shone on this aluminum surface in order to

eject electrons? What is the threshold frequency? What part of the electromagnetic spectrum

is the incident radiation?

m095 01/26

Explain the concept of electrons and holes for conduction in semiconductors.

DO PHYSICS ONLINE 16

m111 06/15

When electromagnetic

radiation shines on metals,

photoelectrons may be

emitted. The maximum

kinetic energy of emitted

photoelectrons is plotted

against radiation frequency

for four metals.

Electromagnetic radiation

of wavelength 187 nm

shines upon an unknown

metal and the maximum

kinetic energy of the

photoelectrons is found to

be 2.5 eV. What is the

unknown metal?

m115 The critical temperature of mercury is 4.2 K. What does this mean? Compare how electric

current is conducted through a sample of mercury at 293 K and 3 K. How does this differ

from the conduction in a p type semiconductor.

m120 03/25

When the laser light was shone onto a photocell, no current was detected. The intensity of the

light was increased but still detected no current. Explain this observation.

m200 04/15

The graph shows the intensity–

wavelength relationship of

electromagnetic radiation

emitted from a blackbody cavity.

In 1900, Planck proposed a

mathematical formula that

predicted an intensity–wavelength

relationship consistent with the

experimental data.

The success of this formula

depended on what hypothesis?

Comment on the curve shown.

DO PHYSICS ONLINE 17

m400 01/24

Sir William Bragg and his son Sir Lawrence Bragg shared the Nobel Prize for physics

in 1915 for their work on X-ray diffraction and crystal structure analysis.

(a) Describe ways in which an understanding of crystal structure has impacted on science.

(b) Outline the methods of X-ray diffraction used by the Braggs to determine the

structure of crystals.

m550 In a modern cathode ray tube (CRO), the electrons are produced by thermionic emission from

a hot filament forming the cathode. The heating of the filament by a low voltage DC power

source causes electrons to be released from the hot filament with nearly zero kinetic energy.

The electrons are then accelerated away from the filament by a large potential difference

applied between the accelerating voltage plates. This forms a narrow electron beam with the

electrons traveling at the same speed and impact onto the fluorescent screen to produce a

bright spot in the centre.

The accelerating voltage was 5.00 kV. When a magnetic field was applied perpendicularly to

the electron beam, the electrons (cathode rays) followed a circular path with a radius of 5.57

mm. Calculate the strength of the magnetic field.

m680 Hertz used a high voltage source to generate a

spark and realised that when another spark was

produced at a receiving coil, energy must have

been transferred. Early in his experiments Hertz

made a chance observation when he could

increase the strength of the spark in the receiving

coils. Which of the following correctly describes

what Hertz had done that led to this observation?

Comment on each alterative. What was the

significance of Hertz’s observation?

(a) He had shone UV light on the receiving coil.

(b) He had slightly increased the gap between the terminals of the receiving coil.

(c) He had placed a glass plate between the transmitter and receiver.

m690 List the properties of the photoelectric effect that could not be explained by classical theory

of electromagnetic waves.

m700 What was the connection between Hertz and the photoelectric effect?

m820 G950

(a) Estimate the temperature of the surface of the Sun, given that the Sun emits light at a

peak wavelength around 550 nm.

(b) Which star is hotter, a blue or a red star. Explain

transmitter

receiver

high

voltage

source

spark

gaps

induction coil

DO PHYSICS ONLINE 18

m950 An electron moving with a velocity of

4.56×106 m.s–1 enters a uniform magnetic field of

strength 3.21×10-2 T at and angle of 42°.

(a) Describe the path of the electron traveling

through the magnetic field.

(b) Calculate the radius of the path followed by

the electron?

m955 03/12

In a first-hand investigation that you performed, you used a

discharge tube containing a Maltese Cross.

What information does the picture tell you?

m960 An electron beam is a special discharge tube is shown.

Show how the electron beam would be deflected by the

magnet. What would happen if the S pole was brought near

the electron beam. Explain your answers.

m964 02/11 Describe the difference between an intrinsic semiconductor and an extrinsic semiconductor.

m965 03/26

Describe Einstein’s contributions to Special Relativity and to Quantum Theory and how these

contributions changed the direction of scientific thinking at the beginning of the 1900s.

uniform magnetic field out of pageB

e42o

N

DO PHYSICS ONLINE 19

m970 The flowchart represents a model of

the scientific method used to show the

relationship between theory and the

evidence supporting it.

Analyse Einstein’s Theory of the

Photoelectric Effect and the evidence

supporting it as an application of this

model of scientific method.

m990 Max Planck and Albert Einstein were both personally affected by the years before, during

and after WWII. They also had strong views about the role science in the period before the

war. Discuss Planck’s and Einstein’s differing views about whether scientific research is

removed from social and political issues.